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well into the nineteenth century.

      However, the scientific pressure to question 4004 BC as the year of creation emerged strongly when Charles Darwin published his theory of evolution by natural selection. While Darwin and his supporters found natural selection compelling, they had to admit that it was a painfully slow mechanism for evolution, wholly incompatible with Ussher’s statement that the world was just six thousand years old. Consequently, there was a coordinated effort to date the age of the Earth by scientific means, with the hope of establishing an age of millions or even billions of years.

      Victorian geologists analysed the rate of sedimentary rock deposition and estimated that the Earth was at least several million years old. In 1897 Lord Kelvin used a different technique: assuming that the world was molten hot when it was formed, he worked out that it must have taken at least 20 million years to cool to its current temperature. A couple of years later, John Joly used a different assumption, namely that the oceans started off pure, and estimated how long it would have taken for the salt to have been dissolved to give the current salinity, which seemed to imply an age of roughly 100 million years. In the early years of the twentieth century, physicists showed that radioactivity could be used to date the Earth, which led to an estimate of 500 million years in 1905. Technical refinements of this technique raised the age to over a billion years in 1907. The dating game was proving to be an enormous scientific challenge, but it was becoming clear that each new measurement was making the Earth appear increasingly ancient.

      As scientists witnessed this huge change in their perception of the Earth’s age, there was a parallel shift in how they viewed the universe. Before the nineteenth century, scientists generally subscribed to the catastrophist view, believing that catastrophes could explain the history of the universe. In other words, our world had been created and shaped by a series of sudden cataclysmic events, such as a massive upheaval of rock to create mountains, or the Biblical flood to sculpt the geological formations that we see today. Such catastrophes were essential for the Earth to have been shaped over the course of just a few thousand years. But by the end of the nineteenth century, after studying the Earth in more detail and in light of the latest results from dating rock samples, scientists moved towards a uniformitarian view of the world, believing in gradual and uniform change to explain the history of the universe. Uniformitarians were convinced that mountains did not appear overnight, but were uplifted at a rate of a few millimetres per year over the course of millions of years.

      The growing uniformitarian movement came to the consensus that the Earth is more than a billion years old, and that the universe must therefore be even older, perhaps even infinitely old. An eternal universe seemed to strike a chord with the scientific community, because the theory had a certain elegance, simplicity and completeness. If the universe has existed for eternity, then there was no need to explain how it was created, when it was created, why it was created or Who created it. Scientists were particularly proud that they had developed a theory of the universe that no longer relied on invoking God.

      Charles Lyell, the most prominent uniformitarian, stated that the start of time was ‘beyond the reach of mortal ken’. This view was reinforced by the Scottish geologist James Hutton: ‘The result therefore of our present enquiry is, that we find no vestige of a beginning, no prospect of an end.’

      Uniformitarianism would have met with the approval of some of the early Greek cosmologists, such as Anaximander, who argued that planets and stars ‘are born and perish within an eternal and ageless infinity’. A few decades later, in around 500 BC, Heraclitus of Ephesus reiterated the eternal nature of the universe: ‘This cosmos, the same for all, was made by neither god nor man, but was, is and always ‘will be: an ever-living fire, kindling and extinguishing according to measure.’

      So, by the start of the twentieth century, scientists were content to live in an eternal universe. This theory, however, was based on quite flimsy evidence. Although there was dating evidence that pointed towards a truly ancient universe, at least billions of years old, the idea that the universe was eternal was largely based on a leap of faith. There was simply no scientific justification for extrapolating from an Earthly age of at least billions of years to a universe that was eternal. Sure enough, an infinitely old universe constituted a coherent and consistent cosmological view, but this was nothing more than wishful thinking unless somebody could find some scientific evidence to back it up. In fact, the eternal universe model was built upon such fragile foundations that it probably deserved the title of myth rather than scientific theory. The eternal universe model of 1900 was almost as flimsy as the explanation that it was the giant blue god Wulbari who separated the sky from the land.

      Eventually, cosmologists would confront this embarrassing state of affairs. Indeed, they would spend the rest of the twentieth century struggling to replace the last great myth with a respectable and rigorous scientific explanation. They strove to develop a detailed theory and sought the concrete evidence to back it up, so that they could confidently address the ultimate question: is the universe eternal, or was it created?

      The battle over the history of the universe, finite or infinite, would be fought by obsessive theorists, heroic astronomers and brilliant experimenters. A rebel alliance would attempt to overthrow an implacable establishment, employing the latest in technology, from giant telescopes to space satellites. Answering the ultimate question would result in one of the greatest, most controversial, most daring adventures in the history of science.

       Chapter 2 THEORIES OF THE UNIVERSE

       [Einstein’s theory of relativity] is probably the greatest synthetic achievement of the human intellect up to the present time.

       BERTRAND RUSSELL

       It is as if a wall which separated us from the Truth has collapsed. Wider expanses and greater depths are now exposed to the searching eye of knowledge, regions of which we had not even a presentiment. It has brought us much nearer to grasping that plan that underlies all physical happening.

       HERMANN WEYL

       But the years of anxious searching in the dark for a truth that one feels but cannot express, the intense desire and the alternations of confidence and misgiving, and the final emergence into light – only those who have experienced it can appreciate it.

       ALBERT EINSTEIN

       It is impossible to travel faster than the speed of light, and certainly not desirable, as one’s hat keeps blowing off.

       WOODY ALLEN

      During the course of the early twentieth century, cosmologists would develop and test a whole variety of models of the universe. These candidate models emerged as physicists gained a clearer understanding of the universe and the scientific laws that underpin it. What were the substances that made up the universe and how did they behave? What caused the force of gravity and how did gravity govern the interactions between the stars and planets? And the universe was made up of space and evolved in time, so what exactly did physicists mean by space and time? Crucially, answering all these fundamental questions would be possible only after physicists had addressed one seemingly simple and innocent question: what is the speed of light?

      When we see a flash of lightning, it is because the lightning is emitting light, which might have to travel several kilometres towards us before reaching our eyes. Ancient philosophers wondered how the speed of light affected the act of seeing. If light travels at a finite speed, then it would take some time to reach us, so by

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